Plastic pipe, such as extruded polyvinyl chloride pipe, is joined together by injection-molded couplings. These couplings are short pieces of plastic with two spaced, in-line, cylindrical openings which are adapted to receive the ends of the pipe to be joined. The internal diameter of the openings make a close fit with the external diameter of the pipe. This pipe is available in diameters ranging from one-half inch to sixteen inches or more. There is a slight taper in the openings from the ends toward the center of the couplings to accommodate slight differences in pipe diameter and to facilitate pipe alignment. The pipe may be bonded to the coupling by a cement or solvent.
These couplings are now made by an injection molding process.
There are several disadvantages to the present process for manufacturing these couplings. Dies used for injection molding are expensive, so that many couplings must be made from a die for the injection molding process to be efficient. A separate die must be made for every diameter and length of coupling. Also, the injection molded material is not as strong and does not resist shock as well as the extruded material.
U.S. Pat. No. 3,495,301 discloses an apparatus for making couplings from extruded material of the kind with which our invention is concerned but it employs a vacuum to form the groove at the center of the coupling. As far as we know, this was never used commercially.